1. Filed of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus capable of reducing, without using a special mechanism, a color misregistration caused by a variation in the load of a belt-shaped intermediate transfer medium or a belt-shaped photoconductor.
2. Description of the Related Art
A color image forming apparatus which uses a belt-shaped intermediate transfer medium or a belt-shaped photoconductor as an image carrier has heretofore been known as a xerographic printer. In a color image forming apparatus which uses a belt-shaped intermediate transfer medium, a latent image of a first color is formed by scanning the surface of a photoconductor with a laser beam, and the latent image is developed to form toner image of the first color and this toner image is transferred to the intermediate transfer medium. Then, a latent image of a second color is formed on the surface of the photoconductor and a toner image of the second color is formed, and this toner image is superimposed on and transferred to the toner image of the first color on the intermediate transfer medium. In this manner, toner images of individual colors Y (yellow), M (magenta), C (cyan) and K (black) are formed on the surface of the intermediate transfer medium in a superimposed state, and the color toner images formed on the intermediate transfer medium are collectively transferred to a recording sheet which is an image carrier.
In a color image forming apparatus which uses a belt-shaped photoconductor, a latent image of a first color is formed by scanning the surface of a photoconductor with a laser beam, and the latent image is developed to form a toner image of the first color. Then, a latent image of a second color is formed on the surface of the photoconductor and a toner image of the second color is formed. In this manner, toner images of plural colors are formed on the surface of the photoconductor in a superimposed state, and the color toner images formed on the photoconductor are collectively transferred to a recording sheet which is an image carrier.
In such a color image forming apparatus, a variation in the speed of the intermediate transfer medium or the photoconductor which is a belt-shaped image carrier causes a variation in the print position of each of the color toner images, resulting in color misregistration or non-uniform density. In general, a variation in the speed of the belt is a periodic variation due to the eccentricity of a belt driving roll, but a variation in speed due to a variation in load acting on the belt is also a large problem. FIG. 13 is a graph showing the relationship between a load acting on the belt-shaped image carrier and the average speed of the belt-shaped image carrier. When the load acting on the belt is small, a small slip or elongation occurs in the belt, and the average speed of the belt linearly decreases with an increase in the load. On the other hand, if the load acting on the belt is a predetermined or more, a slip occurs and it becomes impossible to transport the belt, so that the speed of the belt linearly decreases. In a region in which the average speed linearly varies, if the load increases, the average speed of the belt decreases from V to Vxe2x80x2 as shown in FIG. 14A. In the case of printing at the average speed Vxe2x80x2, since the average speed Vxe2x80x2 is lower than the average speed V, color misregistrations accumulate while the load is increasing, and even if the leading edges of transfer positions of toner images coincide with one another in the sub-scan direction, a color misregistration amount R occurs as shown in FIG. 14B.
On the other hand, a decrease in speed due to the flexure of a driving system such as a driving gear which drives a belt driving roll is different from an average decrease in speed due to a slip or an elongation of the belt, and as shown in FIG. 15A, when a load is applied, the speed instantaneously decreases and is immediately restored. Accordingly, as shown in FIG. 15B, only when a load is applied, a misregistration of a print position instantaneously occurs, and after that no misregistration occurs so that, the misregistration quantities of print positions do not accumulate.
In other words, when a transfer roll or a cleaner to be used during transfer to the belt-shaped intermediate transfer medium makes contact with the belt-shaped intermediate transfer medium, a difference occurs between the speeds of the belt-shaped intermediate transfer medium before and after the transfer roll or the cleaner makes contact with the same, and the difference in speed causes small variations in print position, thus causing a color misregistration of a print position.
The problem of the transfer misregistration also occurs in the case of a drum-shaped photoconductor or intermediate transfer medium.
A general method for ameliorating such small variations in print position is to detect a variation in the speed or the position of a belt-shaped image carrier and correct a print position. Japanese Patent Laid-Open No. 234064/1992 discloses the art of securing an encoder to a roll shaft to be driven by a belt-shaped image carrier and detecting the speed of the belt-shaped image carrier from an angular velocity obtained from the encoder. Japanese Patent Laid-Open No. 175687/1997 describes the art of printing a mark on a belt-shaped image carrier in advance and detecting the mark through a sensor to detect and control the speed of the belt-shaped imager carrier.
However, either of these arts is suited to the art of controlling the speed of a belt-shaped image carrier with high precision, but needs devices such as a detecting device which detects the speed of the image carrier, a computing device which computes a correction amount relative to the detected speed, and a control device which controls a belt driving speed on the basis of the computed result. Either of the arts has, therefore, the problem that the entire size of the image forming apparatus becomes so large as to be disadvantageous in terms of cost and space.
Japanese Patent Laid-Open No. 80853/1997 describes a color image forming apparatus in which a photoconductor and the intermediate transfer medium are driven independently of each other. The color image forming apparatus utilizes an art for correcting a color misregistration due to the fact that a latent image writing part and an intermediate transfer medium are asynchronous with each other. In the art, the phase difference between the rotational period of the latent image writing part and the rotational period of the intermediate transfer medium is detected on the basis of a reference position signal relative to the intermediate transfer medium, and if a color misregistration occurs when a toner image formed on the photoconductor is superimposed on a toner image previously transferred to the intermediate transfer medium, the rotational speed of the intermediate transfer medium is increased or decreased while a toner image is not being transferred to the intermediate transfer medium, thereby correcting the phase difference. Japanese Patent Laid-Open No. 80853/1997 also describes the art of, when a latent image is not being written to the photoconductor, increasing or decreasing the rotational period of the intermediate transfer medium and correcting the phase difference in order to correct a color misregistration occurring in case that the photoconductor and the intermediate transfer medium are driven by identical or different driving devices.
However, the art of Japanese Patent Laid-Open No. 80853/1997 has the problem that since the phase difference is detected on the basis of the reference position signal, a color misregistration or a density deviation cannot be corrected if a variation in load occurs in the intermediate transfer medium or the photoconductor after the reference position signal has been outputted.
The invention has been made to solve the above-described problems, and provide an image forming apparatus capable of reducing a color misregistration due to a variation in the speed of an image carrier such as a photoconductor or an intermediate transfer medium without the need to incorporate a device which detects a variation in the speed of the intermediate transfer medium or the like.
According to a first aspect of the invention, an image forming apparatus superimposes multiple color images on each other on an intermediate transfer medium by rotating a photoconductor and the intermediate transfer medium by using a driving source using an identical driving signal to form a color image, and includes a misregistration correcting part which, when each of the multiple color images is to be transferred to the intermediate transfer medium, increases or decreases a rotational speed of the intermediate transfer medium while no latent image is being formed on the photoconductor to correct a transfer misregistration caused by a variation in load acting on the intermediate transfer medium.
According to the above aspect of the invention, the photoconductor and the intermediate transfer medium are rotated by a driving source using an identical driving signal by driving the photoconductor and the intermediate transfer medium by an identical driving source or by driving the photoconductor and the intermediate transfer medium by different driving sources using an identical driving signal. In the above aspect of the invention, the transfer misregistration is corrected by increasing or decreasing the rotational speed of the intermediate transfer medium while no latent image is being formed on the photoconductor, whereby the respective rotational speeds of the photoconductor and the intermediate transfer medium vary similarly and a difference in speed hardly occurs between the photoconductor and the intermediate transfer medium. Accordingly, even if the rotational speed of the intermediate transfer medium is changed during transfer, a toner image to be transferred is hardly affected.
According to a second aspect of the invention, an image forming apparatus superimposes multiple color images on each other on an intermediate transfer medium by rotating a photoconductor and the intermediate transfer medium by using a driving source using different driving signals to form a color image, and includes a misregistration correcting part which, when each of the multiple color images is to be transferred to the intermediate transfer medium, increases or decreases a rotational speed of the intermediate transfer medium while no image is being transferred to the intermediate transfer medium to correct a transfer misregistration caused by a variation in load acting on the intermediate transfer medium.
According to the above aspect of the invention, the transfer misregistration is corrected by increasing or decreasing the rotational speed of the intermediate transfer medium while no image is being transferred to the intermediate transfer medium, whereby an image to be transferred is not affected. In addition, according to the above aspect of the invention, the photoconductor and the intermediate transfer medium are rotated by using a driving source using different driving signals, whereby even if the rotational speed of the intermediate transfer medium is changed, the rotational period of the photoconductor is hardly affected. Accordingly, even if the rotational speed of the intermediate transfer medium is changed during forming of a latent image, a latent image to be formed on the photoconductor is hardly affected.
According to a third aspect of the invention, an image forming apparatus superimposes multiple color images on each other on an intermediate transfer medium and forms a color image, by rotating a photoconductor and the intermediate transfer medium by using a driving source using identical or different driving signals, and includes a misregistration correcting part which corrects a transfer misregistration caused by a variation in load acting on the intermediate transfer medium, by increasing or decreasing a rotational speed of a light scan device which forms a latent image on the photoconductor, while no latent image is being formed on the photoconductor.
According to the above aspect of the invention, the transfer misregistration is corrected by increasing or decreasing, while no latent image is being formed on the photoconductor, the rotational speed of the light scan device which forms a latent image on the photoconductor by a rotary polygon mirror, whereby an image to be transferred is not affected.
According to a fourth aspect of the invention, an image forming apparatus superimposes multiple color images on each other on an intermediate transfer medium and forms a color image, by rotating a photoconductor and the intermediate transfer medium by using a driving source using identical or different driving signals, and includes a misregistration correcting part which corrects a transfer misregistration caused by a variation in load acting on the intermediate transfer medium, by changing an on-off timing of a light emitting element array which forms a latent image on the photoconductor, while no latent image is being formed on the photoconductor.
According to the above aspect of the invention, the transfer misregistration is corrected by changing, while no latent image is being formed on the photoconductor, the ON timing of the light emitting element array which forms a latent image on the photoconductor by using an LED array and the like, whereby an image to be transferred is not affected.
According to a fifth aspect of the invention, an image forming apparatus superimposes multiple color images on each other on an intermediate transfer medium and forms a color image, by rotating a photoconductor and the intermediate transfer medium by using a driving source using an identical driving signal, and includes a detecting part which detects a phase difference between a timing of forming a latent image on the photoconductor and a rotational speed of the intermediate transfer medium, a phase correcting part which corrects the phase difference by increasing or decreasing a rotational speed of the intermediate transfer medium on the basis of a detection result provided by the detecting part, while no latent image is being formed on the photoconductor, and a misregistration correcting part which corrects, by using the phase correcting part, a transfer misregistration caused by a variation in load acting on the intermediate transfer medium, when each of the multiple color images is to be formed on the intermediate transfer medium.
According to the above aspect of the invention, the photoconductor and the intermediate transfer medium are rotated by a driving source using an identical driving signal, by driving the photoconductor and the intermediate transfer medium by an identical driving source or by driving the photoconductor and the intermediate transfer medium by different driving sources using an identical driving signal. In the above aspect of the invention, the transfer misregistration is corrected by increasing or decreasing the rotational speed of the intermediate transfer medium while no latent image is being formed on the photoconductor, by using the phase correcting part which corrects the phase difference between the timing of forming a latent image on the photoconductor and the rotational period of the intermediate transfer medium. Accordingly, the respective rotational speeds of the photoconductor and the intermediate transfer medium vary similarly and a difference in speed hardly occurs between the photoconductor and the intermediate transfer medium. Accordingly, even if the rotational speed of the intermediate transfer medium is changed during transfer, a toner image to be transferred is hardly affected.
According to a sixth aspect of the invention, an image forming apparatus superimposes multiple color images on each other on an intermediate transfer medium and forms a color image, by rotating a photoconductor and the intermediate transfer medium by using a driving source using different driving signals, and includes a detecting part which detects a phase difference between a timing of forming a latent image on the photoconductor and a rotational speed of the intermediate transfer medium, a phase correcting part which corrects the phase difference by increasing or decreasing a rotational speed of the intermediate transfer medium on the basis of a detection result provided by the detecting part, while no image is being transferred to the intermediate transfer, and a misregistration correcting part which corrects, by using the phase correcting part, a transfer misregistration caused by a variation in load acting on the intermediate transfer medium, when each of the multiple color images is to be transferred to the intermediate transfer medium.
According to the above aspect of the invention, the transfer misregistration is corrected by increasing or decreasing the rotational speed of the intermediate transfer medium while no image is being transferred to the intermediate transfer medium, whereby an image to be transferred to is not affected. In addition, according to the above aspect of the invention, the photoconductor and the intermediate transfer medium are rotated by using a driving source using different driving signals, whereby even if the rotational speed of the intermediate transfer medium is changed, the rotational period of the photoconductor is hardly affected. Accordingly, even if the rotational speed of the intermediate transfer medium is changed during forming of a latent image, a latent image to be formed on the photoconductor is hardly affected.
According to a seventh aspect of the invention, an image forming apparatus superimposes multiple color images on each other on an intermediate transfer medium and forms a color image, by rotating a photoconductor and the intermediate transfer medium by using a driving source using identical or different driving signals, as well as by rotating a latent image forming part, and includes a detecting part which detects a phase difference between a timing of forming a latent image on the photoconductor and a rotational speed of the intermediate transfer medium, a phase correcting part which corrects the phase difference by increasing or decreasing a rotational speed of the latent image forming part on the basis of a detection result provided by the detecting part, while no latent image is being formed on the photoconductor, and a misregistration correcting part which corrects, by using the phase correcting part, a transfer misregistration caused by a variation in load acting on the intermediate transfer medium, when each of the multiple color images is to be transferred to the intermediate transfer medium.
According to the above aspect of the invention, the transfer misregistration is corrected by increasing or decreasing, while no latent image is being formed on the photoconductor, the rotational speed of the latent image forming part, whereby an image to be transferred is not affected.
As to a thirteenth aspect of the invention, according to any one of the first seventh aspects of the invention, the transfer misregistration caused by the variation in load acting on the intermediate transfer medium is calculated from a difference between the rotational period of the intermediate transfer medium when a variation in load is applied thereto and the rotational period of the intermediate transfer medium when no variation in load is applied thereto.
According to an eighth aspect of the invention, an image forming apparatus superimposes multiple color images on each other on a photoconductor and forms a color image, by rotating the photoconductor, and includes a misregistration correcting part which, when a latent image of each of the multiple color images is to be formed on the photoconductor, increases or decreases a rotational speed of the photoconductor while no latent image is being formed on the photoconductor to correct a latent image forming misregistration caused by a variation in load acting on the photoconductor.
According to a ninth aspect of the invention, the eighth aspect of the invention uses a light scan device which forms a latent image on the photoconductor, and a latent image forming misregistration is corrected by increasing or decreasing the rotational speed of the light scan device instead of the rotational speed of the photoconductor. According to a tenth aspect of the invention, the eighth aspect of the invention uses a light emitting element array which forms a latent image on the photoconductor, and a latent image forming misregistration is corrected by changing the on-off timing of the light emitting element array instead of increasing or decreasing the rotational speed of the photoconductor.
According to an eleventh aspect of the invention, an image forming apparatus superimposes multiple color images on each other on a photoconductor and forms a color image, by rotating the photoconductor, includes a detecting part which detects a phase difference between a timing of forming a latent image on the photoconductor and a rotational speed of the photoconductor, a phase correcting part which corrects the phase difference by increasing or decreasing a rotational speed of the photoconductor on the basis of a detection result provided by the detecting part, while no latent image is being formed on the photoconductor, and a misregistration correcting part which corrects, by using the phase correcting part, a latent image forming misregistration caused by a variation in load acting on the photoconductor, when a latent image of each of the multiple color images is to be formed on the photoconductor.
According to a twelfth aspect of the invention, a latent image forming misregistration is corrected by increasing or decreasing the rotational speed of a latent image forming part instead of the rotational speed of the photoconductor used in the above aspect.
According to the eighth to twelfth aspects of the invention, the transfer misregistration is corrected by correcting, while no latent image is being formed on the photoconductor, the latent image forming misregistration by increasing or decreasing the rotational speed of the photoconductor or the light scan device or by changing the on-off timing of the light emitting element array, whereby an image to be transferred is not affected.
According to a fourteenth aspect of the invention, in each of the eighth to twelfth aspects of the invention, the latent image forming misregistration caused by the variation in load acting on the photoconductor is calculated from a difference between the rotational period of the photoconductor when a variation in load is applied thereto and the rotational period of the photoconductor when no variation in load is applied thereto.
According to another aspect of the invention, an image forming apparatus superimposes multiple color images on each other on an intermediate transfer medium and forms a color image, by rotating a photoconductor and the intermediate transfer medium by using a driving source using identical or different driving signals, and includes a part which calculates a transfer misregistration caused by a variation in load acting on the intermediate transfer medium from a difference between a rotational period of the intermediate transfer medium when a variation in load is applied thereto and a rotational period of the intermediate transfer medium when no variation in load is applied thereto, and a misregistration correcting part which corrects the transfer misregistration on the basis of a calculation result provided by the part.
According to another aspect of the invention, an image forming apparatus superimposes multiple color images on each other on a photoconductor and forms a color image, by rotating the photoconductor, and includes a part which calculates a latent image forming misregistration caused by a variation in load acting on the photoconductor from a difference between a rotational period of the photoconductor when a variation in load is applied thereto and a rotational period of the photoconductor when no variation in load is applied thereto, and a correcting part which corrects the latent image forming misregistration on the basis of a calculation result provided by the part.